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  • Title: Caulobacter crescentus synthesizes an S-layer-editing metalloprotease possessing a domain sharing sequence similarity with its paracrystalline S-layer protein.
    Author: Umelo-Njaka E, Bingle WH, Borchani F, Le KD, Awram P, Blake T, Nomellini JF, Smit J.
    Journal: J Bacteriol; 2002 May; 184(10):2709-18. PubMed ID: 11976300.
    Abstract:
    Strains of Caulobacter crescentus elaborate an S-layer, a two-dimensional protein latticework which covers the cell surface. The S-layer protein (RsaA) is secreted by a type I mechanism (relying on a C-terminal signal) and is unusual among type I secreted proteins because high levels of protein are produced continuously. In efforts to adapt the S-layer for display of foreign peptides and proteins, we noted a proteolytic activity that affected S-layer monomers with foreign inserts. The cleavage was precise, resulting in fragments with an unambiguous N-terminal sequence. We developed an assay to screen for loss of this activity (i.e., presentation of foreign peptides without degradation), using transposon and traditional mutagenesis. A metalloprotease gene designated sap (S-layer-associated protease) was identified which could complement the protease-negative mutants. The N-terminal half of Sap possessed significant similarity to other type I secreted proteases (e.g., alkaline protease of Pseudomonas aeruginosa), including the characteristic RTX repeat sequences, but the C-terminal half which normally includes the type I secretion signal exhibited no such similarity. Instead, there was a region of significant similarity to the N-terminal region of RsaA. We hypothesize that Sap evolved by combining the catalytic portion of a type I secreted protease with an S-layer-like protein, perhaps to associate with nascent S-layer monomers to "scan" for modifications.
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